There are exactly 73 primes, beginning with the prime 1093 and ending
with the prime 1613, where 10932 + 10972 + ... + 16132 = 117072.
This is the first instance of a prime number of primes comprising the left member of such an equation. [Haga]

The song "Ride, Captain, Ride" begins with "73 men sailed
up from the San Francisco Bay, ..." [McCranie]

The number formed by the concatenation of odd numbers from 73 down to 1 is prime. [Patterson]

The prime number 73 is the repunit 111 in octal (base 8) and the palindrome 1001001 in binary (base 2). [Dobb]

It took Australian anatomy professor Raymond Dart 73 days to chisel the skull of Taung Child from its surrounding stone matrix. [O'Neil]

Pi Day (March 14 or 3/14) occurs on the 73rd day of the year on non-leap years. Howard Aiken died on Pi Day at the age of 73. He was the primary engineer behind IBM's Harvard Mark I computer. [Rupinski]

Manjul Bhargava showed that if a quadratic form represents all prime numbers up to 73, then it represents all prime numbers. [Poo Sung]

There are 73 composite numbers under 10^2. Proof: 73+25 primes = 98. There are 99 numbers under 10^2, but 1 is neither prime nor composite. This is the only instance of a prime count of composites through 10^22. [Haga]

If the alphabet is converted into numbers (such that a=1,b=2,c=3, etc.), then the word "number" would be 14+21+13+2+5+18 = 73, prime. [Speirs]

The largest two-digit prime n such that n!/reversal(n)+1 is
prime. [Patterson]

A tropical storm is considered a hurricane if its sustained
wind is higher than 73 miles per hour. [Wu]

The Maya, who came to prominence in the New World in the 3rd century A.D., used two calendars, based on a sacred year of 260 days and a vague year of 365 days. The least common multiple of the two calendars, called the calendar round, has 18980 days or 73 sacred years. [Dershowitz]

The success of the magazine known as 73 Amateur Radio
Today gave Wayne Green the money to launch Byte
magazine. [Pierce]

The Discordian calendar has five 73-day seasons: Chaos,
Discord, Confusion, Bureaucracy, and The Aftermath.

73 is the amateur radio operator way of saying "best
regards." [McCranie]

The number of rows in the Arecibo Message graphic. The
Arecibo Message is a radio message that was sent toward a
globular star cluster in Hercules from a radio telescope in
Puerto Rico on November 16, 1974. The entire transmission
lasted a semiprime number of seconds and provided a way for
extraterrestrials to arrange a 73-by-23 array into
recognizable data about our life and solar system.

The first difficult prime to form under the rules of the four 4's puzzle. The problem is to find expressions for numbers using exactly four 4's and a finite number of mathematical symbols and operators in common use. One version of the rules allow only addition, subtraction, multiplication, division, square root, exponentiation, factorial, decimal point, parentheses, as well as concatenation. Here are two possible solutions (achieved by altering the rules above):

Pizza 73 is a Canadian pizza chain. Customers can place orders using the internet.

The largest known prime number p such that
2p does not contain the digit
1. [Capelle]

The largest known prime p such that p -
2k, with p >
2k, is prime or semiprime for
all k > 0. [Capelle]

The largest Scrabble value of "New Mexico." It is the largest possible prime value of all the US States. [Post]

The smallest prime with prime digits that belongs both to
an emirp pair (37, 73) and to a twin prime pair (71, 73) as
the larger member. [Beedassy]

73 is the only emirp that divides the maximum score of New
Super Mario Bros. [Ducayet]

The record number of points scored by any National Football League team in a single game. (Bears beat Redskins 73-0 on December 8, 1940.)

The clepsydra (an ancient device that measured time by
marking the regulated flow of water through a small
opening) had a wheel with 73 teeth. [Robinette]

The smallest prime that is the middle term of three
consecutive numbers each expressible as a sum of two
nonzero squares: 72 = 62 + 62 ; 73 =
32 + 82 ; 74 = 52 +
72. Note that replacing the first prime digit 7
by the prime 23 forms instead the smallest prime (233) that
is the middle term of three consecutive numbers each
expressible as a sum of two distinct nonzero
squares: 232 = 62 + 142 ; 233 =
82 + 132 ; 234 = 32 +
152. [Beedassy]

Starting point of the first Smarandache-Wellin sequence by
minimal decrements to have three primes other than itself,
independent of whether the start is prime (7372717069,
73727170696867, and 737271...232221 are prime). [Merickel]